<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    div {
      width: 100px;
      height: 100px;
      background-color: pink ;
      border: 10px solid #000;
      padding: 20px;
      box-sizing: border-box;
    }
  </style>
</head>
<body>
    <!-- c3盒子模型  好处 加了border和padding 不需要手动减法 -->
    <!-- 用法  box-sizing: border-box -->
  <div >div</div>
</body>
</html>